Ordovician fossils of the Semtín diatreme breccia (Eastern Bohemia, Czech Republic) - A distant analogy of the Prague Basin in the subsurface of the Cretaceous platform?

 

Authors: Fatka O, Budil P, Polechová M, Šilinger M, Kovář V, Valent M Vodička J, Tasáryová Z

Published in: Bulletin of Geosciences, volume 101, issue 1; pages: 123 - 155; Received 10 February 2025; Accepted in revised form 2 April 2026; Online 3 May 2026

Keywords: Trilobita, Bivalvia, organic-walled microfossils, Late Ordovician, East Bohemia, Czech Republic,

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Abstract

Ordovician rocks east of the Prague Basin are very rarely exposed because they are largely concealed beneath the platform sediments of the Bohemian Cretaceous Basin. A unique record of Ordovician strata occurs at Pardubice-Semtín, where Palaeozoic xenoliths were brought to the surface by a nephelinitic basalt diatreme that pierced the Mesozoic cover. In 1896, Jaroslav Jiljí Jahn briefly reported a fossil assemblage from these xenoliths, including machaeridians, hyolithids, brachiopods, molluscs, and trilobites, which closely resembles the fauna of the Late Ordovician Vinice and Zahořany formations of the Prague Basin. Most of Jahn’s material was long considered lost but it was rediscovered in 2020. Its thorough revision, including the first study of associated acritarchs and chitinozoans, refines the age and stratigraphic affinity of the fauna. A continuous distribution of Ordovician rocks between the Prague Basin, the bedrock of the Bohemian Cretaceous Basin, and the Železné hory Mountains area has been traditionally assumed. Differences in faunal composition, lithology, and metamorphism, along with detected and inferred tectonic faults, suggest a discontinuity between the Ordovician basement beneath the south-eastern part of the Bohemian Cretaceous Basin and the Ordovician of the Železné hory Mountains area. Even though there are numerous similarities between fossils at Pardubice-Semtín and that of the Prague Basin, we consider them more likely to represent distinct, albeit likely communicating, basins within the Teplá-Barrandian Unit.